Numerous methods have been developed for producing particulate titanium dioxide in general and pigmentary titanium dioxide in particular. Of the numerous methods developed, only two have gained any significant commercial importance. These two methods include the older so-called "sulfate process" and the more recent chloride process, both of which were developed specifically for use in the manufacture of pigmentary titanium dioxide. Of these two processes, the chloride process has become the process of choice by reason of the various advantages it offers over the sulfate process. One advantage of this process is that plants designed around this process are more economical to operate. Another advantage of the chloride process is that when utilized to manufacture pigmentary titanium dioxide, such pigments are cleaner, whiter and more durable than the titanium dioxide pigment products manufactured by the older sulfate process. One particularly important advantage of the chloride process over the sulfate process is that the former is significantly more environmentally acceptable than the latter.
Despite the aforementioned advantages of the chloride process, this particular process is not well suited to producing titanium dioxide pigment having an anatase crystalline structure and which structure is highly desirable in the titanium dioxide pigments utilized in the manufacture of such products as paper, fibers, ceramics and the like. Although chloride process produced titanium dioxide pigments which typically possess a rutile crystalline structure, can be used in these applications, such use often is undertaken with some measure of compromise. In other instances, however, such as in the delustering of various synthetic fibers, only titanium dioxide pigment of the anatase crystalline structure can be used. This is due, in the main, to the lower abrasivity inherent in titanium dioxide pigments having this particular crystalline structure.
Because of the need for pigmentary titanium dioxide of the anatase crystalline structure, the use of the sulfate process continues in spite of the environmental problems associated therewith. To remedy these environmental problems, which principally arise from the massive evolution of sulfuric acid gases and immense quantities of waste dilute sulfuric acid which are generated in the process, numerous solutions have been proposed. For example, two solutions which have been proposed and largely implemented throughout this industry to cope with the large quantities of waste dilute sulfuric acid produced are neutralization with calcium carbonate (which produces a calcium sulfate waste product) and concentration of the dilute acid to 95 to 98 percent sulfuric acid for reuse in the process. However, both of these solutions give rise to problems of their own. For instance, utilization of the first solution, i.e., neutralization, results typically in about one and three quarters parts by weight of waste calcium sulfate being produced for each part by weight of waste dilute acid (or equivalent amount of sulfate ions) neutralized, creating a serious disposal problem in its own right. Likewise, concentration of the waste dilute acid is quite costly in terms of energy, since about nine parts by weight of water must be evaporated for every part by weight of concentrated acid produced. Additionally, the quality of such concentrated acid generally is unacceptable for reuse in the process. This is due mainly to impurities such as chromium and manganese (resulting from the action of sulfuric acid or the original titanium bearing ore employed in the sulfate process) present in the concentrated acid and which impurities detrimentally effect the quality of the final pigment product.
From the above, it becomes clear that a need exists for a method by which pigmentary titanium dioxide products, particularly pigmentary titanium dioxide having the anatase crystalline structure can be produced and which method overcomes or avoids altogether the disadvantages inherent in the conventional sulfate process. The present invention provides such a method. Furthermore, the method of this invention has the versatility of not only being capable of producing pigmentary titanium dioxide having an anatase crystalline structure, but also titanium dioxide having the rutile crystalline structure.